1. Field of the Invention
The present invention relates to a sheet-position detection device for detecting the position of a conveyed sheet, and an image forming apparatus, such as a copier, a facsimile apparatus, a printer, a composite apparatus having the functions of these apparatuses, or the like, which includes the sheet-position detection device in the main body thereof, for forming an image on a sheet.
2. Description of the Related Art
Some conventional image forming apparatuses are configured such that, in order to form an image at an appropriate position, for example, at a central position, in a direction crossing a sheet conveying direction, the image forming position can be moved in a direction crossing the sheet conveying direction. In such an image forming apparatus, the position of a conveyed sheet in a direction crossing a sheet conveying direction is detected by a sheet-position detection device mounted in the main body of the image forming apparatus before the image formation location, and the image forming position is adjusted and moved in a direction crossing the sheet conveying direction based on position information.
Particularly, in recent image forming apparatuses, in order to improve productivity during duplex image formation, a sheet is re-conveyed to an image forming portion for image formation on a second surface after image formation on a first surface without performing regulation in a direction crossing a sheet conveying direction, such as lateral alignment in a stack-type intermediate tray. Hence, the position of the sheet in a direction crossing the sheet conveying direction varies due to various factors, and it is sometimes impossible to accurately form an image at the appropriate position.
Accordingly, in order to transfer an image at the appropriate position on the second surface of the sheet, a technique to detect the position of an edge of the sheet in a reconveyance path is indispensable.
In order to solve the above-described problem, recent sheet-position detection devices are mostly provided at an upstream position near an image forming portion in order to detect a sheet which has been conveyed from a sheet feeding tray and on a first surface of which an image is not yet formed, as well as in image formation on a second surface, and improve accuracy in position in image formation. Furthermore, detection of an edge of a sheet is performed for a sheet being conveyed (moving), so as not to degrade efficiency in sheet conveyance.
Sheet-position detection devices are grossly classified into two types of devices, i.e., contact-type devices, each including a detection flag contacting an edge of a sheet, and means for detecting the operation of the detection flag when contacting the sheet, and a non-contact-type devices for detecting an edge of a sheet using a light-transmitting-type sensor instead of directly contacting the edge.
Sheets include ordinary paper, thin resin sheets serving as a substitute for ordinary paper, thick paper, postcards, labels and the like.
Recent image forming apparatuses have higher conveying speeds as a result of pursuit of higher productivity. The pursuit of higher productivity is required for all sheet sizes. At the same time, requests for an improvement in the stability and accuracy of the image forming position on a sheet are increasing.
However, the above-described conventional sheet-position detection devices sometimes cannot respond to the recent request for a higher speed. That is, a sheet being conveyed at a high speed vibrates considerably. Hence, particularly in a sheet-position detection device using a contact-type detection flag which directly touches an edge of the sheet, the detection flag is pushed by the sheet more than necessary, thereby sometimes causing a degradation of accuracy in detection, even causing erroneous detection. In addition, the amount of wear of a portion of the detection flag contacting an edge of a sheet conveyed at a high speed increases as the conveying speed increases. Accordingly, contact-type sheet-position detection devices cannot respond to a recent request for high durability, for example, because of extreme degradation in detection accuracy by a detection flag after the lapse of limit period of durability, and damage of the detection flag after wear proceeds.
Furthermore, when detecting a sheet which is short in the conveying direction during high-speed conveyance, the time for detection becomes shorter, resulting in further difficulty in detection. In order to solve such a problem, it is necessary to increase the speed of the operation and the control of a detection mechanism, irrespective of use of a contact type or a non-contact type, resulting in a increase in the cost of the sheet-position detection device.
The present invention has been made in consideration of the above-described problems.
It is an object of the present invention to provide a sheet-position detection device in which accuracy in sheet-position detection is improved while achieving high productivity, high durability, and reduction in the production cost, and an image forming apparatus in which an image is formed on a predetermined position of a sheet according to sheet-position information of the sheet-position detection device.
According to one aspect, the present invention which achieves the above-described object relates to a sheet-position detection device including means for temporarily stopping a sheet conveyed along a sheet conveying path, and sheet-position detection means for detecting a position of the sheet in a direction crossing a conveying direction of the sheet during the stoppage of the sheet.
In one embodiment, the stop means includes a pair of rotating members for rotating in order to convey the sheet while grasping the sheet.
In another embodiment, the sheet-position detection means detects an edge of the sheet parallel to the sheet conveying direction.
In still another embodiment, the sheet-position detection means includes a detection flag rotatable by contacting the edge of the sheet parallel to the sheet conveying direction, and a sensor for detecting rotation of the detection flag.
In yet another embodiment, the detection flag is provided at a moving member movable in the direction crossing the sheet conveying direction, and the position of the sheet is calculated based on a distance of the moving member moved until the detection flag covers the sensor.
According to another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a sheet mounting unit for mounting sheets, image forming means for forming an image on a sheet supplied from the sheet mounting unit so as to be adjustable in a direction crossing a direction of conveying the sheet, the above-described sheet-position detection device, and image-formation control means for determining a position of formation of the image based on sheet-position information from the sheet-position detection device.
According to still another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a sheet mounting unit for mounting sheets, image forming means for forming an image on a sheet supplied from the sheet mounting unit so as to be adjustable in a direction crossing a direction of conveying the sheet, skew correction means, positioned between the sheet mounting unit and the image forming means, for correcting skew of the sheet supplied from the sheet mounting means by temporarily receiving the sheet, the above-described sheet-position detection device, and image-formation control means for determining a position of formation of the image based on sheet-position information from the sheet-position detection device. The skew correction means also operates as the stop means of the sheet-position detection device.
According to yet another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a sheet mounting unit for mounting sheets, image forming means for forming an image on a sheet supplied from the sheet mounting unit so as to be adjustable in a direction crossing a direction of conveying the sheet, a reversal guiding channel for guiding the sheet by turning the sheet so as to form an image on a surface opposite to a surface where the image has been formed by the image forming means, skew correction means, provided in the reversal guiding channel, for correcting skew of the sheet subjected to reversal guiding by temporarily stopping the sheet, the above-described sheet-position detection device, and image-formation control means for determining a position of formation of the image based on sheet-position information from the sheet-position detection device. The skew correction means also operates as the stop means of the sheet-position detection device, and the sheet-position detection means of the sheet-position detection device is provided at the skew correction means.
In the sheet-position detection device of the present invention, since the detection of the position of the sheet in the direction crossing the conveying direction is performed while the sheet temporarily stops, it is possible to improve accuracy in sheet-position detection while achieving high productivity, high durability, and reduction in the production cost.
Since the image forming apparatus of the present invention includes the sheet-position detection device having high accuracy in sheet-position detection in the main body of the apparatus, it is possible to accurately and assuredly form an image at a predetermined position of a sheet.
According to the present invention, by bending the reversal path provided in the conveying unit so as to be separated from the sheet mounting unit, and causing the conveying path to join with the reversal path at the bent portion, it is possible to perform stable sheet conveyance while achieving reduction in the size of the image forming apparatus.